because now she is

>Dany has been a terrible person. We have cheered her on
>killing people for a very long time. We've watched her enjoy
>killing people with a real bloodlust. We've cheered her on
>because we thought the killings were justified. So it's not a
>huge leap to think she is justified in burning down king's
>landing since she has made the decision to rule by fear. If
>you plan to rule by fear, what better way then burn down a
>city.